@Alchemist, I think the advice “just do it” is good, but to be a little more specific about how to go about this—for me, it helps to have a ritual to begin the writing process. This puts you in the frame of mind where you’re less inclined to slack off or procrastinate.
My “ritual” is: I set aside some time to write, make a cup of nice coffee, and then sit down in a comfy chair or my bed with my nice headphones (to shield from distractions) and write for at least a couple of hours. I’m sure the physical effects of caffeine help, but it’s more than that—once you start doing this routine over and over again it’s easier to slip into the “writing mentality.”
If you have a laptop, you might also try going to a coffee shop (with headphones). Sometimes I find I write better at a coffee shop. I think it’s because there are people around who can “watch” me—which I would internalize as judging me if I slack off—but not people who will actually bother me and distract me.